Scanning probe microscopy (SPM) is an important tool for performing measurements at the nanoscale in imaging bacteria or proteins in biology, as well as in the electronics industry. An essential element of SPM is a sharp, stable tip that possesses a small radius of curvature to enhance spatial resolution. Existing techniques for forming such tips are not ideal. High-aspect-ratio, monolithically integrated, as-grown carbon nanofibers (CNFs) have been formed that show promise for SPM applications by overcoming the limitations present in wet chemical and separate substrate etching processes.
The CNFs of this innovation have been synthesized in a load-lock-based DC PECVD (plasma-enhanced chemical vapor desposition) growth chamber, where the CNF growth was done on Si substrate with high-purity acetylene (C2H2) and ammonia (NH3) at 700 °C. The ratio of C2H2:NH3 = [1:4], which has been determined to minimize the amount of amorphous carbon on the substrate during growth. When the desired growth pressure was attained (3–15 Torr), a DC glow discharge was ignited, and growth was continued for a fixed duration. The PECVD growth parameters, such as growth pressure, catalyst thickness, and plasma power, were varied to see their impact on the physical characteristics of the CNFs (e.g., diameter and length).
The mechanical characteristics of the CNFs were measured in a custom-built in-situ mechanical deformation instrument, the SEMentor, comprising a scanning electron microscope (SEM) and the nanoindenter. This instrument has generally been used to explore uniaxial deformation and defect evolution in individual, metallic pillars formed by using the focused-ion-beam (FIB), for example.
Bending tests were performed with a nanoprobe that deflected an individual CNF, and provided insight into their mechanical resilience in shear. In-situ electrical measurements were then conducted on individual, as-grown CNFs using a nanomanipulator probe stage mounted inside an SEM (FEI Quanta 200F) that was equipped with an electrical feed-through. Tungsten probes were used to make the two-terminal electrical measurements of individual, vertically oriented, as-grown CNFs with an HP4156C parameter analyzer.
For SPM applications, stress concentrators may exist at the CNF-to-substrate interface, as well as within the body. In-situ uniaxial compression tests were performed on arrays of CNFs inside the SEMentor, which provided some insight into the nature of the mechanical bond between the CNF and substrate. A Berkovich tip, which is a pyramidal, shallow-angled tip, was used to indent the forest of CNFs. The SEM image taken after indentation revealed that the CNFs fractured within the tube body rather than at the CNF-to-substrate interface, where a fracture angle αf ≈ 25°–35° (relative to the CNF or central axis) was computed.
The significance of αf was correlated to the structural characteristics of the CNFs, which were deciphered from transmission electron microscopy (TEM) that was performed with FEI Tecnai-F20 Scanning –(S) TEM, with a field emission source of 200 kV. The TEM analysis of the mechanically transferred CNFs grown directly on Si revealed a herringbone structure where the graphite basal planes were inclined to the central axis at a cone angle α, where α ≈ 30°. Since αf and α did not differ appreciably, the nanomechanical measurements performed in the SEMentor confirm that the CNFs sheared from within the basal planes of the CNFs, indicating that the adhesion of the CNFs to the substrate was very strong. In addition, the image in the figure shows the CNFs can tolerate a large degree of mechanical strain where bending angles φ as large as 70° could be accommodated elastically, confirming the promise such carbon-based nanostructures have for SPM applications..